LL_ALLOCATED_SPACE and other considerations

Question

I have a kernel module wherein I capture a packet in PRE-ROUTING hook for some processing. I then allocate a new skb(cant do it in the same skb) and put the processed payload of the input skb and the ip header in this new skb. I would then want to do a netif_rx for this new skb and let it traverse the kernel networking stack.

I am little confused with the size of the new skb I should allocate, where my skb->data should point to (to network_header or mac header). What should be the skb->len, should it consider mac header or not?

len; // total length of new ip datagram
skb_new = dev_alloc_skb(len + LL_ALLOCATED_SPACE(skb->dev) + ETH_LEN);

after this how much should I reserve for LL header and trailer and where should my skb_new->data point to.

I want to call netif_rx(skb_new) after filling in the required details in skb. Basically what should follow after allocating the skb and before calling netif_rx. Any link or description will help.

Thanks in advance.

Answer 1

A socket buffer (skb) holds ALL of the information for the current protocol data unit (PDU) and accounts for ALL of the data being passed around for this PDU. Regardless of how much slack space you leave at the beginning of the skb, you point your skb->data to the beginning of YOUR data.